1. Field of the Invention
This invention relates to a process and a relevant device for the introduction of liquid samples into apparatuses for gas chromatographic analysis, specially designed for samples containing high percentages of solvent and namely samples having relatively large volume, containing compounds to be analysed in very low concentrations in the relevant solvent.
As well known, the introduction of samples containing high percentages of solvent compared to the compounds to be separated involves serious problems due to the need of eliminating a considerable portion of solvent before it reaches the detector.
2. Description of the Prior Art
Several techniques of separation or splitting have been proposed, wherein only a part of the sample is fed to the detector. The latest techniques envisage to enrich the sample in the components to be analyzed, eliminating a considerable portion of the solvent alone before its introduction into the separation column.
According to said techniques, the liquid sample is injected into a vaporization "chamber" (such as for instance the "retention gap" in K. Grob et al., J. Chromatogr., 295 (1982) 15) where the liquid solvent evaporates and thus separates from the compounds. By this technique it is possible to avoid the loss of volatile compounds, but the system is sensitive to contaminations caused by high boiling compounds and water.
Another technique used is the one known as "solvent split injection", Vogt et al, in J. Chromatogr. 174 (1979) 437. If volatile compounds are present, this method presents some drawbacks, one of which in particular makes its application difficult. It is in fact necessary to accurately regulate the speed of introduction of the liquid sample into the vaporization chamber, which must be approximately the same as the speed of evaporation of the solvent, which speed on its turn depends on the exhaust flow, on the temperature and the solvent. As a matter of fact, in case the evaporation speed of the solvent is higher than that of sample feeding, even the most volatile compounds will mostly vaporize and will be discharged through the exhaust valve without being analyzed. If, on the contrary, the sample is fed to the chamber at a much higher speed than that of the solvent evaporation, some still liquid sample will remain inside the vaporization chamber, from where, after exceeding a critical volume of approximately 20 or 30 microliters, it will enter the column and the exhaust line with all the known problems involved.
It is furthermore necessary to accurately control the time for opening and closing the exhaust valve, in order to avoid both the presence of a big volume of liquids inside the injector (when the discharge duct is closed too soon) and the loss of most volatile compounds (when the discharge duct is closed too late).
It is evident how complex the regulation of discharge conditions is, even considering the impossibility of exactly measuring the flowrate of vapours in the splitting duct and their dilution in the carrier gas.